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The invention relates to a variable-focus lens comprising two opposing
walls (21, 22) which are which define a space containing an isolating
liquid (31) on the first wall (22) and a conductive liquid (32) which
covers the isolating liquid and which comes into contact with the second
wall (21). The surface (24) of the first wall (22) has high wettability
for the isolating liquid and low wettability for the conductive liquid,
while the surface (23) of the second wall (21) has high wettability for
the conductive liquid and low wettability for the isolating liquid.

1. A variable-focus lens comprising: first and second transparent
insulating plates (21, 22) placed facing each other orthogonally to the
axis of the lens; in a space defined between said plates, an insulating
liquid (31) on the first plate (22), and a conducting liquid (32)
covering the insulating liquid and coming into contact with the second
plate (21), the surface (24) of the first plate (22) having high
wettability for the insulating liquid and low wettability for the
conducting liquid, and the surface (23) of the second plate (21) having
high wettability for the conducting liquid and low wettability for the
insulating liquid; and a conducting plate (26) deposited on the first
plate and provided with a central opening in which the insulating liquid
is placed, the conducting plate being, on the inside of the cell, coated
with an insulating coating (27), the wettability characteristics of which
are optimized for obtaining the desired miniscus shape at rest between
the insulating liquid and the conducting liquid.

2. The lens as claimed in claim 1, in which the surface of at least one of
the walls is coated with a layer (23, 27) giving it the desired
wettability characteristics.

3. The lens as claimed in claim 1, in which the wettability of the surface
(24) of the first wall (22) is such that the contact angle of a drop of
the insulating liquid in the conducting liquid for this surface is less
than 75.degree..

4. The lens as claimed in claim 1, in which the wettability of the surface
(23) of the second wall (1, 11, 21) is such that the contact angle of a
drop of the conducting liquid in the insulating liquid for this surface
is less than 75.degree..

5. A variable-focus lens assembly comprising, between an upper transparent
plate (21) and a lower transparent plate (22), a pair of liquids (31, 32)
having an interface, the profile of which can be modified by varying a
voltage applied between one of the liquids, a conducting one, and an
electrode close to the other liquid, the insulating one, characterized in
that it includes a first conducting ring (50), in electrical contact with
the conducting liquid and bearing on the upper plate, and a second
conducting ring (40), in electrical contact with said electrode and
bearing on the lower plate, each of these rings having a cylindrical
region (52, 42), the opposed faces of these cylindrical regions being
threaded, and a third ring (70), which is insulating, having internal and
external threads, being interposed between said two cylindrical parts.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to variable-focus lenses and more
particularly to such lenses employing the deformation of a drop of liquid
by the phenomenon of electrowetting.

PRIOR ART

[0002] Various embodiments of variable-focus lenses are described in the
Applicant's European patent 1166157. FIG. 1 of the present application
substantially reproduces FIG. 12 of that patent. A cell is defined by two
insulating plates 1 and 2 and sidewalls (not shown). The lower plate 2,
which is not plane, includes an indentation or recess 3 that receives a
drop of an insulating liquid 4. The rest of the cell is filled with an
electrically conducting liquid 5, which is immiscible with the insulating
liquid, has a different refractive index but substantially the same
density. An annular electrode 7, open facing the recess, is placed on the
rear face of the lower plate 2. Another electrode 8 is in contact with a
conducting liquid 5. Through the phenomenon of electrowetting, it is
possible, depending on the voltage V applied between the electrodes 7 and
8, to modify the curvature of the contact surface between the two
liquids, which for example passes from the concave initial shape denoted
by the reference 4 to the convex shape shown by the dotted line and
denoted by the reference 9. Thus, a beam of light passing through the
cell orthogonally to the plates 1 and 2 in the region of the drop 4 will
be focused at a longer or shorter distance depending on the applied
voltage.

[0003] Other embodiments of variable-focus lenses are described in
document WO-A-03/069380. FIG. 2 of the present application substantially
reproduces FIG. 1 of that document. A cell is defined by two insulating
plates 11 and 12 and a cylindrical sidewall 13. The lower part 2 of the
cell receives a drop 14 of an insulating liquid. The rest of the cell is
filled with a conducting liquid 15, which is immiscible with the
insulating liquid and has a different refractive index, but substantially
the same density. The cylindrical sidewall 13 contains an annular
electrode 17. Another electrode 18 formed on the internal face of the
upper plate 11 is in contact with the conducting liquid 15. It is
possible, depending on the voltage V applied between the electrodes 17
and 18, to modify the curvature of the contact surface between the two
liquids, which for example passes from the concave initial shape denoted
by the reference 14 to the convex shape, shown by the dotted line and
denoted by the reference 19. Thus, a light beam passing through the cell
orthogonally to the plates 11 and 12 in the region of the drop 14 will be
focused at a longer or shorter distance depending on the applied voltage.

[0004] Although these solutions are satisfactory, they do have drawbacks
in their implantation. This is because in practice the conducting liquid
is generally an aqueous liquid and the drop, generally of an oily liquid,
is positioned when the cell has been totally immersed beforehand in the
aqueous liquid. The oily drop is injected via the bottom of the cell, but
it may be difficult to expel the water from the bottom of the cell. The
result may be that the drop is poorly positioned or that a drop of water
is trapped beneath the drop of oil, introducing unacceptable optical
perturbations.

[0005] Thus, it is necessary to take certain operating precautions in
order to position the drop correctly, this drop being liable to divide
and adhere partly to each of the plates 1 and 2 or 11 and 12. In the case
of FIG. 1, the drop may also end up partly above the plane upper part of
the lower plate 2.

[0006] These drawbacks also occur when the lens is used in an environment
subject to shocks hence, under the effect of an acceleration, the drop
may have a tendency to deform or be displaced, and to depart from the
housing provided for this purpose. It may then be very tricky, if not
impossible, to reposition it correctly.

SUMMARY OF THE INVENTION

[0007] One object of the present invention is to mitigate these drawbacks,
so as to make it easier to carry out the drop implantation operations,
and to prevent excessively large displacements of the liquids resulting
in the drop being out of position or divided.

[0008] Another object of the present invention is to provide a particular
variable-focus lens cell structure enabling its parameters to be
optimized.

[0009] To achieve these objects, and others, the present invention
provides a variable-focus lens comprising, in a space defined between two
facing walls orthogonal to the axis of the lens, an insulating liquid on
a first of the walls and, covering the insulating liquid and coming into
contact with the second wall, a conducting liquid. The surface of the
first wall has high wettability for the insulating liquid and low
wettability for the conducting liquid and the surface of the second wall
has high wettability for the conducting liquid and low wettability for
the insulating liquid.

[0010] According to one embodiment of the present invention, the surface
of at least one of the walls is coated with a layer giving it the desired
wettability characteristics.

[0011] According to one embodiment of the present invention, the
wettability of the surface of the first wall is such that the contact
angle of a drop of the insulating liquid in the conducting liquid for
this surface is less than 75.degree..

[0012] According to one embodiment of the present invention, the
wettability of the surface of the second wall is such that the contact
angle of a drop of the conducting liquid in the insulating liquid for
this surface is less than 75.degree..

[0013] According to one embodiment of the present invention, the wall
bearing the insulating liquid comprises a plate of insulating material
coated with a material having high wettability for the insulating liquid
and low wettability for the conducting liquid, a conducting plate with a
central opening, in which the insulating liquid is placed, being
deposited on said insulating plate, this conducting plate being coated,
on the inside of the cell, with one or more materials suitable for
providing insulation and having optimum wetting properties for the
operation of the lens.

[0014] Another subject of the present invention is a variable-focus lens
assembly comprising, between an upper transparent plate and a lower
transparent plate, a pair of liquids having an interface, the profile of
which can be modified by varying a voltage applied between one of the
liquids, a conducting one, and an electrode close to the other liquid,
the insulating one, this assembly including a first conducting ring, in
electrical contact with the conducting liquid and bearing on the upper
plate, and a second conducting ring, in electrical contact with said
electrode and bearing on the lower plate, each of these rings having a
cylindrical region, the opposed faces of these cylindrical regions being
threaded, and a third ring, which is insulating, having internal and
external threads, being interposed between said two cylindrical parts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The objects, features and advantages, together with others, of the
present invention will be explained in detail in the following
description of particular embodiments, given as nonlimiting examples, in
relation to the appended figures in which:

[0016] FIGS. 1 and 2, described above, are sectional views of examples of
variable-focus lenses according to the prior art, which illustrate
problems that the invention seeks to solve;

[0017] FIG. 3 is a sectional view of one embodiment of a lens according to
the invention; and

[0018] FIG. 4 is a sectional view of an example of a lens assembly for the
lens of FIG. 3.

DETAILED DESCRIPTION

[0019] FIG. 3 shows one embodiment of a variable-focus lens cell according
to the present invention. The cell is bounded by two plates 21 and 22
made of transparent insulating materials, which are orthogonal to the
optical axis of the lens. According to a fundamental aspect of the
present invention, which also applies to cells of the types illustrated
in FIGS. 1 and 2, these plates consist of, or are coated on their
internal walls with, materials having specific wettability properties,
namely the internal wall of the upper plate 21 that has to be in contact
with the conducting liquid has high wettability for this conducting
liquid and low wettability for the insulating liquid and the internal
wall of the lower plate 22 that has to be in contact with the insulating
liquid has wettability characteristics the reverse of the above.

[0020] In the standard case in which the conducting liquid is an aqueous
liquid and the insulating liquid is an oily liquid, the material or
coating of the upper plate will be highly hydrophilic, while that of the
lower plate will be highly hydrophobic.

[0021] In FIG. 3, a hydrophilic coating has been denoted by the reference
23 and a hydrophobic coating by the reference 24.

[0022] In practice, various materials with hydrophilic properties may be
used. Good results have been obtained using, as hydrophilic surface, a
mineral glass or a silicon-oxide-coated polycarbonate.

[0023] Among hydrophobic materials, perfluoro-ethylene-propylene (FEP) and
perfluoroalkoxy (PFA) may be especially mentioned. These materials, which
are transparent, may constitute the wall itself. Other materials may also
be used, but only as coating layer. Polymers such as soluble
fluoropolymers may especially be mentioned, for example the products sold
under the reference AF 1600 by DuPont de Nemours, or the products sold by
Cytonix, or else layers of silanes grafted onto the surface of the
materials, which give the latter desirable wettability properties.

[0024] In practice, the wettability of the surface of a wall is measured
by the angle that the contact surface between the two fluids makes to the
surface of the wall. The contact angle is defined as the angle between
the contact surface between two fluids and the solid wall on which this
contact surface bears. This angle depends only on the nature of the
fluids in question and on the wall, and not the shape of the wall. If the
two fluids are two immiscible liquids, identical wettability for the two
liquids means that the contact surface is substantially perpendicular to
the surface of the wall. If the surface of the wall exhibits greater
wettability for one of these liquids, this means that the contact surface
between the two liquids makes an acute angle on the side with the liquid
in question.

[0025] Thus, a surface may be termed hydrophobic if the peripheral surface
of the drop makes an obtuse angle to the surface of the wall on the drop
side. Conversely, if this angle is an acute angle, the surface is
considered to be hydrophilic. As a general rule, the wettability of a
surface is measured although two different liquids are present, the
interface surface between the two liquids forming the periphery of the
drop.

[0026] Given below is a table summarizing the representative wettability
contact angles of certain materials used as regards hydrophobic and
hydrophilic surfaces, in combination with the liquid forming the
characteristic drop and the surrounding fluid (gas or liquid). These
tests were carried out using, as water, deionized ultrapure water, and,
as oil, an oil sold by Exxon Mobil Chemical under the name ISOPAR V (CAS
No. 64742-46-47). The glass used as hydrophilic surface was an optical
glass, such as that denoted by the acronym BK7.
TABLE-US-00001
Liquid Surrounding Contact
Coating drop liquid angle
Hydrophobic AF1601 Water Air 105.degree.
surface (DuPont) Oil Air 57.degree.
Oil Water <10.degree.
801-A Water Air 104.degree.
(Cytonix) Oil Air 65.degree.
Oil Water 45.degree.
Hydrophilic Glass Water Air <10.degree.
surface

[0027] In practice, good results are obtained when the wettability of the
surface 24 of the plate receiving the oil drop is such that the contact
angle of the oil in the water is less than 750 and the wettability of the
surface 23 of the plate in contact with the aqueous liquid is such that
the contact angle of the water in the oil is less than 750.

[0028] In the exemplary embodiment shown in FIG. 3, the lower part of the
cell comprises the abovementioned insulating plate 22, the upper surface
(or a coating) 24 of which has the desired wettability characteristics. A
plate 26 of a conducting material coated with an insulating coating 27 is
laid on, preferably bonded to the insulating plate. The insulating
coating 27 may be a multilayer coating. Its external surface is such that
its wettability characteristics are optimized for obtaining the desired
meniscus shape at rest between the insulating liquid 31 and the
conducting liquid 32. Thus, according to one advantage of this
embodiment, the wettability properties of the coating of the upper part
24 of the insulating plate 22 can be dissociated from the wettability
properties of the external face of the insulating coating 27. The
properties of the layer 23 are mainly intended to optimize the adhesion
of the drop to its support, and the properties of the layer 27 are mainly
intended to optimize the characteristics of the electrowettability
function.

[0029] FIG. 4 illustrates an example of an assembly of the various plates
of the lens of FIG. 3. A first metal ring 40 has a lower part in the form
of a plate 41 that supports the lower insulating and conducting plates
22, 26 and ensures that there is electrical conductivity with the
conducting material of the plate 26, and a cylindrical peripheral part 42
that surrounds the structure and is provided with an internal thread 44.
A second metal ring 50 has an upper part in the form of a plate 51, which
bears on the upper insulating plate 21 and ensures that there is
electrical contact with the conducting liquid 32, preferably via
conductive coating 53 provided on the periphery of the plate 21. The
second metal ring 50 thus has a cylindrical peripheral part 52 provided
with an external thread 54. The cylindrical peripheral part 52 is
internal to the cylindrical peripheral part 42. A first cylindrical
insulating ring 60 is placed between the external periphery of the lens
and the internal periphery of the second conducting ring 50. A second
cylindrical insulating ring 70 is placed between the cylinders 42 and 52
and is provided with internal and external threads that are respectively
intended for coupling with the external thread 54 of the cylinder 52 and
the internal thread 44 of the cylinder 42. An O-ring seal 80 keeps the
plate 26 separated from the plate 21. The height is also maintained by
the length of the ring 60.

[0030] This structure is assembled in the following manner. The cell 21-27
is fitted with the seal 80, the oil 31 and the water 32 being in place.
Next, the following are fitted in succession: the ring 40, the ring 70
screwed into this ring 40, the ring 60, and the ring 50 screwed into the
ring 70 so as to clamp the seal 80 and to maintain sealing. The
conducting rings 40 and 50 may be provided with threads or drillholes for
the insertion of connection elements and/or for mounting the lens on the
device to which it has to be fitted.

[0031] Of course, this merely shows one possible example of an assembly,
and various alternative embodiments will be apparent to those skilled in
the art. In particular, the various rings may be provided with locking
shoulders. The seal 80 and the insulating ring 60 may be placed
differently or even combined provided that the function of isolating the
rings 40 and 50, the sealing function and the function of keeping the
upper and lower plates of the lens spaced apart are ensured. In addition,
this type of assembly may be suitable for variable-focus lenses other
than that described in relation to FIG. 3.

[0032] As indicated above, the present invention also applies to the
exemplary embodiments shown in FIGS. 1 and 2. In these embodiments, the
materials of the plates 1 and 11, or of the internal coatings on these
plates, will be such that the internal walls of these plates have
strongly hydrophilic properties. In the case of FIG. 2, the material of
the lower plate 12, or a coating on this plate, will have highly
hydrophobic properties. In the case of FIG. 1, a highly hydrophobic
coating will preferably be provided at the bottom of the recess 3, a
coating optimized for the operation of the lens on the oblique walls of
the recess, and a hydrophilic coating on the upper plane peripheral part
of the plate 2, in order to prevent the oil drop from adhering thereto.

[0033] Moreover, FIGS. 1 and 3 show regions for positioning the insulating
liquid that are in the form of a cup with plane sidewalls inclined at
450. Various other shapes could be used, for example cylindrical or toric
shapes.

[0034] It follows from the structure according to the present invention
that the insulating drop will naturally tend to be positioned in the
highly hydrophobic region of the bottom of the recess in the case of
FIGS. 1 and 3, and of the lower plate 12 in the case of FIG. 2, and that,
even if the drop accidentally comes into contact with another surface,
such as the surface of the upper plate, it will not adhere to this
surface and will naturally be repositioned on the lower face side.

[0035] To give an example, in the case of FIG. 3, the lower part of the
opening in the plate 26 may have a diameter of 3 to 5 mm, this plate
having a thickness of a few tenths of an mm, the distance between the
plate 26 and the plate 21 also being a few tenths of an mm, and the upper
and lower plates being glass plates with a thickness of the order of 1
mm.